Optical communication systems are well known in the art. The need to switch sometimes from one traffic carrying path to another arises in any one of a number of instances, e.g. when there is an occasional fault occurring in one of the traffic carrying channels due to failing components, or when an expansion of the operating communication network is required, etc. Therefore, an auxiliary channel is usually incorporated in such systems, allowing the diversion of traffic being transmitted along one path, to another path. The latter path is sometimes referred to as a protection path/link. Conventional optic fibers based networks have implemented 1:1 redundancy for the optical links extending within the network, which relies on a certain amount of automatic switching. In these systems, when a loss of signal (to be referred to hereinafter as “LOS”) or alarm indication signal (“AIS”) conditions are noted for a link connecting two locations, a diversion of the traffic being transmitted takes place to the available redundant path. This diversion enables continuing with the transmission of data between such two locations.
In addition, with the evolving of optical networks, there is a growing need for using optical amplifiers. An optical amplifier is a device that amplifies an optical signal directly, without the need to first have it converted to an electrical signal, have it amplified, and then convert the amplified electrical signal to an optical signal for further conveying the latter along the transmission path. An optical amplifier may be considered as a laser device without an optical cavity, or one in which feedback from the cavity is suppressed.
Optical link protection usually incorporates a 1×2 or 2×2 optical switches in order to enable switching from the main path to the protected path as described above, when the use of the protection mode is required, e.g. when a fiber-cut occurs.
U.S. Pat. No. 7,551,857 describes an optical amplifier which may be quickly returned from a shutdown state to a regular operating mode after getting recovery information of a fault. In involves gain setting by Amplified Spontaneous Emission (“ASE”) to the repeaters on the up-stream side during the shutdown state, and outputting ASE light with the same intensity as the Wave Division Muliplexed (“WDM”) signal. Accordingly, before realizing the recovery of a shutdown, the gain setting is completed with the light whose intensity is within the safe criterion. After realizing the recovery of the shutdown, the optical transmission system can be returned quickly to the regular operating mode.
For an optical network which comprises a ring structure carrying a bi-directional optical data signal, US 20020181037 discloses a method for affecting failure protection method by which when no signal is detected at a primary pre-amplifier located at one of the network hubs (used for pre-amplifying a primary optical signal received from a first direction along the ring structure), the primary pre-amplifier is shut down and a secondary pre-amplifier, which is located at the one network hub for pre-amplifying a redundant optical signal corresponding to the optical signal and received from the opposing direction along the ring structure, is powered up.
However, when switching to the protected path, there is a concern that the link Optical Fiber Amplifiers (OFA's) or other active elements will be shutdown during the diversion of the traffic from the main optical link to the protection link, which in turn might result in a too long recovery period for these elements (e.g. say about 100 ms). In addition, different parts of the protected link require different shutdown rules.
Therefore, a method is required for OFA's shutdown that addresses these requirements, in order to guarantee a fast protection switching of less than about 50 ms.